This invention pertains to pumps of the roto-dynamic type which employ impellers rotatable in a housing; they, generally, are either of the centrifugal or the axial-flow embodiments, and in particular to a novel impeller for such roto-dynamic or propeller-type pumps.
The centrifugal pump comprises an impeller which consists of a hub and at least one covering disc which carries a plurality of vanes; such are called open impellers. A closed impeller has two covering discs, with the vanes therebetween. In both cases liquid is drawn axially into the center of the impeller, and leaves the latter essentially in a tangential direction from the circumference of the impeller.
The axial-flow pump is of an open type, having a space between the vanes and the enclosing or surrounding housing. It differs from the centrifugal pump in that the liquid moves, essentially, in an axial direction through the pump. The tangentially-directed velocity vector which is obtained after the impeller is reduced in a number of guide vanes in the housing downstream of the impeller. Said guide vanes also normally serve as support elements in the housing structure.
When pumping waste water, and certain types of industrial-process water containing elongated fibers, the operation can be disturbed by rag shreds, fibers, etc., getting stuck onto the leading edges of the impeller vanes as well as on the guide vanes of the pump housing. The build-up thereof can dramatically decrease the efficiency of the pump. The result will normally be that the flow decreases and the power demand increases. One way to make the collected debris to leave the vanes is to let the impeller rotate in reverse, at certain intervals, but this is not a solution which recommends itself. Another way in which to diminish the likelihood of pump clogging by such debris is to incorporate a cutting means for comminuting the fibers etc., before they are ingested into the impeller. A device of this sort is shown in the Swedish patent No. 8205774-6. It has the disadvantage, however, that the cutting means quickly wears out and the clogging problems may become worse.
It is also known to design the vanes with backward swept leading edges in the flow direction, whereby the pollutions more easily glide off. An example is shown in the European published publication 237 921. This impeller has however a design which deteriorates the cavitation abilities.
Most propeller pumps on the market today are designed with a hub of a spherical shape provided with vanes which are turnable around axes mainly perpendicular to the direction of the rotation axis. This possibility to control the vanes means that a wide range of flow capacity is covered with one and the same pump. The spherical form also means that a vane may be turned into different angles while keeping the same slot towards the hub thus minimizing losses in the slot.
When designing an axial-flow pump it is often desired to keep a high specific rotation speed, i.e., a maximum flow should be obtained at a given speed. This means that the inlet area, the area between the hub and the wall of the housing, should be maximized. As the outer diameter of the housing is limited because of the cavitation problem, there is only a decrease of the diameter of the hub left.
Spherical formed hubs always mean problems of a flow technical art when the radius is shortened as the possible geometric length of the connection between the hub and the vanes also shortens. If an acceptable efficiency should be obtained, said connection length must never be less than a certain value and this means that there must be a compromise between the two goals: large flow and high efficiency, respectively.
For pump impellers where the vanes should be adjustable, it is desired that the entire vane is kept collected around the axis around which the vane is turned when adjusted. Then a minimal axial translation movement occurs during rotation and a flow effective change of angles for all profile sections (chords) is obtained. If, for a conventional propeller pump having a relatively high specific rotation speed, the profile sections that form the vane are swept backwards in such a way that the leading edges become self-cleaning, the performance of the pump will be almost unchanged, provided that the angle is kept. However, short root connections between the hub and the vanes means that the trailing edge, also, will be markedly swept backwards; consequently, there is no optimum turning axis. The swept impeller will, therefore, be less effective after turning to another angle than would be the non-swept impeller. The aforesaid means that it is impossible to design a turnable and swept vane having an optimum performance, if a spherically-formed hub is used.
In order to have a vane which is sufficiently collected around its turning axis, and simultaneously is swept backwards, it is a known practice to shorten the chord lengths in the direction of the periphery of the vane. This limits the backward sweep of the trailing edge of the vane. The effect of this, however, is that the cavitation abilities deteriorate.
The spherically-formed hub could be avoided by molding the hub and vanes in one, single piece. Such, however, does not give the same flexibility and is expensive.